1. Field of the Invention
The present invention relates to a diffractive optical element used for a plurality of wavelengths or light in a predetermined wavelength band and a picture taking apparatus using the same and, more specifically, a diffractive optical element used for part of a picture taking optical system for forming a color image by using color light bands of three or more colors.
2. Related Background Art
As a method of correcting the chromatic aberration of an optical system, a method of combining two glass materials (lenses) having different dispersions is available.
In contrast to this method of reducing chromatic aberration by combining these two glass materials, a method of reducing chromatic aberration by providing a diffractive optical element (to be also referred to as a diffraction grating hereinafter) having a diffraction effect for a lens surface or a portion other than the lens surface of an optical system is disclosed in, for example, SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Patent Application Laid-Open Nos. 4-213421 and 6-324262 and U.S. Pat. No. 5,044,706. This method uses the physical phenomenon that when the refractive powers of a refraction plane and diffraction plane in an optical system are equal in sign, chromatic aberrations occur in opposite directions with respect to a ray having a given reference wavelength. Such a diffractive optical element can be made to have an aspherical lens effect by arbitrarily changing the period of the periodic structure of the element, and hence exhibits a great effect in reducing monochromatic aberration.
When a given ray is refracted, the ray remains a single ray after refraction, whereas when a given ray is diffracted, the ray is split into diffracted light components of a plurality of orders. When, therefore, a diffractive optical element is to be used for an optical system, a grating structure must be determined to make light beams in an operating wavelength range concentrate on a specific order (to be also referred to as a xe2x80x9cdesign orderxe2x80x9d hereinafter). When light concentrates on a specific order, the intensity of diffracted light components of other orders is low. When the intensity of a diffracted light component is 0, the light component does not exist.
To exploit the characteristics of the above diffractive optical element, the diffraction efficiency of a ray of the design order must be sufficiently high in the entire operation wavelength range. A ray of a diffraction order other than the design order is formed into an image at a position different from that of a ray of the design order, and hence becomes flare (light). In an optical system using a diffractive optical element, therefore, careful consideration must be given to the spectral distribution of the diffraction efficiencies of rays of the design order of the diffractive optical element and the behaviors of rays of orders other than the design order (unnecessary diffracted light components).
FIG. 14 shows the diffraction efficiency characteristics of rays of specific diffraction orders in a case where a diffractive optical element 1 having a single-layer diffraction grating 6 formed on a substrate 4 as shown in FIG. 13 is formed on a given surface in an optical system. In the following description, a diffraction efficiency value is the ratio of the light intensity of each diffracted light component to that of the entire transmitted light beam without any consideration to reflected light at a grating interface surface because of an increase in complexity in description. Referring to FIG. 14, the abscissa represents the wavelength; and the ordinate, the diffraction efficiency. This diffractive optical element is designed such that the diffraction efficiency of the first diffraction order (the solid line in FIG. 14) is maximized in the operating wavelength range. That is, the design order is the first order. FIG. 14 also shows the diffraction efficiencies of diffraction orders (first order xc2x1first order, i.e., the zero order and second order) close to the design order.
As shown in FIG. 14, the diffraction efficiency of the design order is maximized at a given wavelength (to be referred to as a xe2x80x9cdesign wavelengthxe2x80x9d hereinafter) and gradually decreases at other wavelengths. A decrease in the diffraction efficiency of this design order corresponds to diffracted light of another order and becomes flare. When a plurality of diffractive optical elements are used, in particular, a decrease in diffraction efficiency at a wavelength other than the design wavelength leads to a decrease in transmittance.
Various conventional arrangements for reducing the influence of flare have been proposed.
For example, the diffractive optical element disclosed in Japanese Patent Application Laid-Open No. 9-127322 is obtained by optimally selecting three different materials (three diffraction gratings 6, 7, and 12) and two different grating thicknesses d1 and d2 and arranging the respective diffraction gratings close to each other with an equal pitch distribution, as shown in FIG. 15. With this arrangement, as shown in FIG. 16, high diffraction efficiency at the design order is realized throughout the entire visible region.
The present inventor has also proposed an arrangement capable of suppressing a decrease in diffraction efficiency in Japanese Patent Application Laid-Open No. 10-133149. The diffractive optical element proposed in this reference has a cross-sectional shape of a multilayer formed by stacking two layers, and high diffraction efficiency of a design order is realized throughout the entire visible region by optimizing the refractive indices, dispersion characteristics, and grating thicknesses of materials for two layers 6 and 7, as shown in FIG. 17.
Japanese Patent Application Laid-Open No. 10-104411 discloses a diffractive optical element configured to decrease the intensity of unnecessary diffracted light components of orders near the design order by properly shifting the design wavelength by adjusting the grating thickness of a kinoform type diffractive optical element like the one shown in FIG. 13.
The present applicant has also proposed an optical system in Japanese Patent Application No. 11-344369 (Japanese Patent Application Laid-Open No. 2000-241614), which is configured to properly reduce unnecessary diffracted light components of orders near the design order by using a diffractive optical element having a multilayered structure.
Of the conventional elements, the diffractive optical element proposed in Japanese Patent Application Laid-Open No. 9-127322 is configured to greatly improve the diffraction efficiency of the design order, and hence the intensity of unnecessary diffracted light components as diffracted light components of orders other than the design order is reduced, thus decreasing flare. However, color flare is noticeable in an obtained image. Furthermore, no detailed description is made about the color appearance of flare, the intensity of flare, and the like.
In Japanese Patent Application Laid-Open No. 10-104411, the influences of color flare of unnecessary-order light on a grating shape having one diffraction plane as shown in FIG. 13 is described (to be referred to as a xe2x80x9csingle-layer DOExe2x80x9d hereinafter). However, no description is made about flare in a diffractive optical element having a cross-sectional shape of a multilayer obtained by stacking two or more layers (to be referred to as a xe2x80x9cmultilayered DOExe2x80x9d hereinafter).
In an optical system using the above multilayered DOE, flare is greatly reduced as compared with a single-layer DOE. However, unnecessary diffracted light is not completely eliminated but is slightly left.
In applications to optical systems in which picture taking (projection) conditions remain unchanged (e.g., the reader lens of a copying machine and the projection lens of a liquid crystal projector), the influences of flare are suppressed by multilayered DOEs to a level at which no problem arises. However, according to various studies made by the present inventor, it was found that in optical systems designed to take pictures of objects in various conditions, e.g., a camera and video camera, slight residual flare might pose a problem.
For example, if a high-luminance light source exists in an object field, picture taking operation is performed at proper exposure with respect to an object other than the light source instead of setting proper exposure with respect to the light source. Assume that the light source is exposed to light 1,000 times higher in intensity than that for proper exposure. In this case, even if residual flare is as small as 0.2%, since the intensity of flare from the light source portion is multiplied by 1,000, the resultant flare has an intensity twice as higher than that of proper exposure, resulting in noticeable flare in a taken image.
As described above, when a multilayered DOE is applied to a camera or video apparatus, even slight flare may raise a problem. If a flare component has wavelength dependence, in particular, color flare similar to color light characteristics based on the single-layer DOE disclosed in Japanese Patent Application Laid-Open No. 10-104411 is produced even in an arrangement using a multilayered DOE.
It is an object of the present invention to provide an diffractive optical element in which color flare caused by unnecessary-order diffracted light is made unnoticeable, and a picture taking apparatus having the diffractive optical element.
In order to achieve the above object, in one aspect of the present invention, there is provided a picture taking apparatus having a diffractive optical element, which forms an image on picture taking means by using the diffractive optical element in an optical system, the diffractive optical element having a grating structure obtained by stacking a plurality of diffraction gratings made of at least two type of materials with different dispersion properties to enhance diffraction efficiency for a particular design order throughout a predetermined wavelength region and a plurality of design wavelengths which exist in a predetermined design order and at which a maximum optical path length difference in the grating structure becomes an integer multiple of a wavelength, and the diffractive optical element having a design wavelength xcex0 satisfying conditions given below:
U1,n(xcex0)xe2x88x92U1,r(xcex0) greater than 0.75[U3,n(xcex0)xe2x88x92U3,rxe2x88x921(xcex0)]
xe2x80x83U1,n(xcex0)xe2x88x92U1,r(xcex0) greater than U3,n(xcex0)xe2x88x92U3,r(xcex0)
U1,n(xcex0)+U1,r(xcex0) greater than 0.5[U2,n(xcex0)xe2x88x92U2,rxe2x88x921(xcex0)]
U1,n(xcex0)xe2x88x92U1,r(xcex0)xe2x89xa7U2,n(xcex0)xe2x88x92U2,r(xcex0) 
(arbitrary integer satisfying O less than r less than n)
U1,n(xcex0), U2,n(xcex0), and U3,n(xcex0) being given by       U1    ,                  n        ⁡                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              n                                            m            +            n                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁡                                      (                    λ                    )                                                  ⁢                                  T                  ⁡                                      (                    λ                    )                                                  ⁢                                  F1                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U2    ,                  n        ⁡                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              n                                            m            +            n                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁡                                      (                    λ                    )                                                  ⁢                                  T                  ⁡                                      (                    λ                    )                                                  ⁢                                  F2                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U3    ,                  n        ⁡                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              n                                            m            +            n                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁡                                      (                    λ                    )                                                  ⁢                                  T                  ⁡                                      (                    λ                    )                                                  ⁢                                  F3                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                          
and, U1,r(xcex0), U2,r(xcex0), and U3,r(xcex0) being given by       U1    ,                  r        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              r                                            m            +            r                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F1                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U2    ,                  r        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              r                                            m            +            r                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F2                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U3    ,                  r        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              r                                            m            +            r                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F3                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                          
for
∫Dm(xcex0,xcex)L(xcex)T(xcex)F1(xcex)dxcex=∫Dm(xcex0,xcex)L(xcex)T(xcex)F2(xcex)dxcex=∫Dm(xcex0,xcex)L(xcex)T(xcex)F3(xcex)dxcex
where
Dp(xcex0, xcex): a diffraction efficiency of a wavelength xcex when a diffraction order of the diffractive optical element is represented by p and a design wavelength is represented by xcex0,
Dm(xcex0, xcex): a diffraction efficiency of the wavelength xcex when the diffraction order of the diffractive optical element is represented by m and the design wavelength is represented by xcex0,
m: a design order,
n: an order (n greater than 0) contributing to flare,
xcexa3: an addition symbol; although xcexa3 in each equation states that values from P=mxe2x88x92n to P=m+n are added, p=m is not included,
∫: an integration symbol,
xcex1 and xcex2: a shortest wavelength and longest wavelength in a predetermined wavelength region,
Sp(xcex): a contribution ratio at the wavelength xcex of p-order diffracted light in a specific area on a light-receiving surface (an imaging position of a design order),
L(xcex): a spectral characteristic at the wavelength xcex of a light beam incident on the optical system,
T(xcex): a transmittance of the optical system at the wavelength xcex, and
F1(xcex), F2(xcex), and F3(xcex): spectral sensitivity characteristics of light-receiving means each serving to detect light in a given wavelength range in the picture taking means; a wavelength at which the spectral sensitivity becomes highest increases in the order of F1(xcex), F2(xcex), and F3(xcex).
In one aspect of the present invention, the plurality of diffraction gratings stacked on each other preferably include at least one diffraction grating whose direction of an increasing or decreasing of a grating thickness differs from that of remaining diffraction gratings.
In one aspect of the present invention, the predetermined wavelength region is preferably a visible region.
Further, in one aspect of the present invention, the diffraction gratings are preferably stacked on a substrate, and when the stacked diffraction gratings are named as a first diffraction grating, a second diffraction grating, and an ith diffraction grating, starting from the diffraction grating close to the substrate, the first diffraction grating and the substrate are preferably made of the same material.
Further, in one aspect of the present invention, the optical system preferably comprises an imaging optical system.
Further, in another aspect of the present invention, there is provided a diffractive optical element which has a grating structure obtained by stacking a plurality of diffraction gratings made of at least two types of materials with different dispersion properties to enhance diffraction efficiency for a particular design order throughout a predetermined wavelength region and a plurality of design wavelengths which exist in a predetermined design order and at which a maximum optical path length difference in the grating structure becomes an integer multiple of a wavelength, and makes flare caused by diffracted light of an order other than the design wavelengths have a bluish hue like white tinted with blue.
Further, another aspect of the present invention preferably makes flare caused by diffracted light of an order other than design orders have a bluish hue like white tinted with blue.
Further, in another aspect of the present invention, an optical system preferably comprises the above diffractive optical element.
Further, in another aspect of the present invention, a picture taking apparatus preferably images an object on picture taking means by using the above optical system.
Further, in another aspect of the present invention, there is provided a diffractive optical element used in an optical system to form an image on picture taking means, the diffractive optical element having a grating structure obtained by stacking a plurality of diffraction gratings made of at least two types of materials with different dispersion properties to enhance diffraction efficiency for a particular design order throughout a predetermined wavelength region and a plurality of design wavelengths which exist in a predetermined design order and at which a maximum optical path length difference in the grating structure becomes an integer multiple of a wavelength, and the diffractive optical element having a design wavelength xcex0 satisfying conditions given below:
U1,n(xcex0)xe2x88x92U1,r(xcex0) greater than 0.75[U3,n(xcex0)xe2x88x92U3,rxe2x88x921(xcex0)]
U1,n(xcex0)xe2x88x92U1,r(xcex0) greater than U3,n(xcex0)xe2x88x92U3,r(xcex0) 
U1,n(xcex0)xe2x88x92U1,r(xcex0) greater than 0.5[U2,n(xcex0)xe2x88x92U2,rxe2x88x921(xcex0)]
U1,n(xcex0)xe2x88x92U1,r(xcex0)xe2x89xa7U2,n(xcex0)xe2x88x92U2,r(xcex0) 
(arbitrary integer satisfying O less than r less than n)
U1,n(xcex0), U2,n(xcex0), and U3,n(xcex0) being given by       U1    ,                  n        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              n                                            m            +            n                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F1                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U2    ,                  n        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              n                                            m            +            n                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F2                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U3    ,                  n        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              n                                            m            +            n                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F3                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                          
and, U1,r(xcex0), U2,r(xcex0), and U3,r(xcex0) being given by       U1    ,                  r        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              r                                            m            +            r                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F1                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U2    ,                  r        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              r                                            m            +            r                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F2                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                                U3    ,                  r        ⁢                  (                      λ            ⁢                          xe2x80x83                        ⁢            0                    )                    =                        ∑                      p            =                          m              -              r                                            m            +            r                          ⁢                              ∫            λ1            λ2                    ⁢                                    {                                                L                  ⁢                                      (                    λ                    )                                                  ⁢                                  T                  ⁢                                      (                    λ                    )                                                  ⁢                                  F3                  ⁡                                      (                    λ                    )                                                  ⁢                                  Dp                  ⁡                                      (                                                                  λ                        ⁢                                                  xe2x80x83                                                ⁢                        0                                            ,                      λ                                        )                                                  ⁢                                  Sp                  ⁡                                      (                    λ                    )                                                              }                        ⁢                          ⅆ              λ                                          
for
∫Dm(xcex0,xcex)L(xcex)T(xcex)F1(xcex)dxcex=∫Dm(xcex0,xcex)L(xcex)T(xcex)F2(xcex)dxcex=∫Dm(xcex0,xcex)L(xcex)T(xcex)F3(xcex)dxcex
where
Dp(xcex0, xcex): a diffraction efficiency of a wavelength xcexwhen a diffraction order of the diffractive optical element is represented by p and a design wavelength is represented by xcex0,
Dm(xcex0, xcex): a diffraction efficiency of the wavelength xcex when the diffraction order of the diffractive optical element is represented by m and the design wavelength is represented by xcex0,
m: a design order,
n: an order (n greater than 0) contributing to flare,
xcexa3: an addition symbol; although xcexa3 in each equation states that values from P=mxe2x88x92n to P=m+n are added, p=m is not included,
∫: an integration symbol,
xcex1 and xcex2: a shortest wavelength and longest wavelength in a predetermined wavelength region,
Sp(xcex): a contribution ratio at the wavelength xcex of p-order diffracted light in a specific area on a light-receiving surface (an imaging position of a design order),
L(xcex): a spectral characteristic at the wavelength xcex of a light beam incident on the optical system,
T(xcex): a transmittance of the optical system at the wavelength xcex, and
F1(xcex), F2(xcex), and F3(xcex): spectral sensitivity characteristics of light-receiving means each serving to detect light in a given wavelength range in the picture taking means; a wavelength at which the spectral sensitivity becomes highest increases in the order of F1(xcex), F2(xcex), and F3(xcex).
Further, in the another aspect of the present invention, the plurality of diffraction gratings stacked on each other preferably include at least one diffraction grating whose direction of a grating of a grating shape differs from that of remaining diffraction gratings.
Further, in the another aspect of the present invention, the predetermined wavelength region is preferably a visible region.
Further, in the another aspect of the present invention, the diffraction gratings are preferably stacked on a substrate, and when the stacked diffraction gratings are named as a first diffraction grating, a second diffraction grating, and an ith diffraction grating, starting from the diffraction grating close to the substrate, the first diffraction grating and the substrate are preferably made of the same material.